This invention relates to the microfinishing of surfaces, and particularly to the reduction of minute discontinuities or asperities on the surface of discs used in the recording and reproduction of data.
As hard disc recording and reproduction systems for digital data processing have evolved, there have been continuing increases in track density and longitudinal recording density, such that data recording capacities have increased by orders of magnitude. A fundamental factor in achieving these results has been the development of transducers which are supported by air bearings at very small flying heights (1 microinch or less) above the surface of the disc. The aerodynamics of the pad facing the disc, and a sensitive and precise gimbal support arm, facilitate noncontact operation with these minute gaps which in turn provides extremely efficient coupling between the transducer and the active surface (whether magnetic or magneto-optical) of the disc.
As advances have been made in these respects, corollary advances have also been made in disc manufacture, and in manufacturing processes and test procedures, to enable the disc surfaces to be essentially planar, to a high degree of precision. The discs are mass produced, enabling the virtually universal adoption of hard disc files for data processors in small and large capacity systems to become feasible because of the very low cost and very high performance levels which have been reached. The discs may be single sided or double sided, as they are burnished and finished to a given smoothness. With submicroinch flight heights, however, burnishing alone is not satisfactory, because very minor irregularities, typically called asperities, still can exist. These must either be eliminated before the disc can be installed, or the disc must be rejected for use.
Automated test beds have been devised for use in a final honing procedure for these high capacity hard discs. These test beds include xe2x80x9cglide headxe2x80x9d mechanisms, each glide head having a sensitive force sensor so that, with the glide head flying above the disc surface at a given height (at the order of a microinch or less) asperities can be detected. The disc is rotated at angular velocities typical for normal operations, giving surface rates of 400 ips to 600 ips, depending upon radial position. The glide head is scanned across the active recording surface of the disc, with the sensor generating a signal excursion whenever an asperity is encountered. Depending on the amplitude and duration of the signal excursion, these asperities can be categorized (as for example xe2x80x9chardxe2x80x9d hits or xe2x80x9csoftxe2x80x9d hits) and the instrumentation system can identify the radius for future processing. At this point, a honing head, supported by a gimbal arm to be radially movable, is also scanned across the disc, specifically to those radial positions at which asperities had been detected. The honing head typically has small projections from a flat surface, and flat contact areas on the projections that are separated by grooves, such that the edges of the projections engage and hone the asperities.
Such honing head designs, however, have disadvantages that become most apparent when attempting to provide a surface for recording/reproduction head operation at submicroinch levels. Highly polished flat surfaces in contact induce forces of molecular attraction between them, and thus introduce stiction effects which can render operation non-uniform. In addition, with a honing head of this design the top increment of an asperity may be separated from its base, but is not necessarily removed from the disc itself, thus representing an object that can possibly interfere with signal transduction.
After honing head operation, the disc is again tested by the glide head and instrumentation system, to verify that asperities beyond the chosen threshold have been eliminated. Thereafter, the disc can be approved for use in a production unit. These processes are carried out in clean room conditions and the discs are confined within closed environments with air properly filtered to remove all but very minute particle sizes which can be tolerated.
Devices, systems and methods in accordance with the invention utilize the trailing edges of an array of recesses formed in a planar like surface having an anti-stiction coating and including a plurality of channels leading from the recesses to the radially outward portion of the honing head. The recesses preferably converge toward the trailing edge, with straight or curved sides, and shear off the apices of asperities, the channels causing particulate matter to flow radially outwardly from the honing head and to be urged by centripetal forces off the disc. Air trapped under the leading edge of the honing head merely raises its leading edge minutely, but the angle assists the shearing action by the subsequent trailing edges. Concurrently, the recesses in the outwardly angled channels are under negative pressure relative to the surrounding ambient pressure, drawing the asperities from the surface of the disc and toward the airflow region adjacent the honing head that is created by the high speed disc.
In a more particular example of devices and methods in accordance with the invention, useful in eliminating asperities so that recording heads can fly at altitudes of 0.5 microinch or less, the recesses are essentially diamond shaped trenches, each having a trailing edge defined by converging sides of the diamond. Each trench of this type has dimensions of about 168 microns by 168 microns, and a depth of 10 to 20 microns, while the overall configuration of the pad is 40 mm by 80 mm in this practical example. The honing head is mounted at the end of a gimbal arm, and urged toward the disc with a pressure which depends upon head area, in this instance about 6.5 grams. The channels extend at an angle toward the trailing edge and are a depth corresponding to the trenches, and so angled that they intercept each of a series of intervening trenches along the path to the external radius. The surface of the honing head incorporates a diamond like coating (dlc) which hardens the surface, improves the shearing action and enhances uniformity of operation because it substantially reduces the stiction effect. Alternatively, superior results have also been achieved with trenches that are curved, e.g. oval, in configuration, and the advantages are also preserved in other geometries, such as triangular trenches which have an apex at the trailing edge.
Honing heads of this character may readily be fabricated using photolithographic techniques, as by depositing a photosensitive resist material on a planar head, exposing the photoresist material through a photographic mask to leave a removable pattern corresponding to the trenches and channels, and then removing the material by ion milling, acid etching or plasma etching to form straight sides for shearing, and recesses of the chosen depth. After the honing head has been processed to this extent, the dlc material is applied, readying the head for attachment in the assembly in use.